As microchip technology continues to increase in complexity and decrease in component size, dimensions are shrinking to the quarter micron-scale and smaller. With use of the current high-yield photolithographic techniques, the margin of error has become increasingly tighter such that a single misaligned fabrication step can cause an entire chip to be flawed and be discarded. As devices shrink further, overstepping each process step's window of error increases the likelihood of fabrication failure. A production-worthy device feature requires incidental skill of a process engineer and a fabrication operator to fabricate the feature.
One device that is subject to the ever-increasing pressure to miniaturize is the dynamic random access memory (DRAM). DRAMs comprise arrays of memory cells that contain two basic components—a field effect access transistor and a capacitor. Typically, one side of the transistor is connected to one side of the capacitor. The other side of the transistor and the transistor gate electrode are connected to external connection lines called a bit line and a word line, respectively. The other side of the capacitor is connected to a reference voltage. Therefore, the formation of the DRAM memory cell comprises the formation of a transistor, a capacitor and contacts to external circuits.
It is advantageous to form integrated circuits with smaller individual elements so that as many elements as possible may be formed in a single chip. In this way, electronic equipment becomes smaller, assembly and packaging costs are minimized, and integrated circuit performance is improved. The capacitor is usually the largest element of the integrated circuit chip. Consequently, the development of smaller DRAMs focuses to a large extent on the capacitor. Three basic types of capacitors are used in DRAMs—planar capacitors, trench capacitors, and stacked capacitors. Most large capacity DRAMs use stacked capacitors because of their greater capacitance, reliability, and ease of formation. For stacked capacitors, the side of the capacitor connected to the transistor is commonly referred to as the “storage node,” and the side of the capacitor connected to the reference voltage is called the cell plate. The cell plate is a layer that covers the entire top array of all the substrate-connected devices, while there is an individual storage node for each respective storage bit site.
The areas in a DRAM to which an electrical connection is made are the gate of a transistor of the DRAM, a contact plug to an active area, and the active area itself. Active areas, which serve as source and drain regions for transistors, are discrete specially doped regions in the surface of the silicon substrate. A bit line contact corridor (BLCC) is created in order to make electrical connection to an active area. The BLCC is an opening created through the insulating material separating the bit line and the active area. The BLCCs are filled with a conductive material, such as doped polysilicon, doped Al, AlSiCu, or Ti/TiN/W. Before filling the BLCC, however, a process engineer must design a process flow for fabricating the BLCC that assures that the BLCC is not misaligned and, therefore, not prone to shorting out or subject to errant charge leaking due to an exposed cell plate in the BLCC.
Conventional methods of fabricating bit line contacts may tend to cause shorting of the bit line contact in the BLCC into the cell plate due to misalignment. For example, titanium is conventionally sputtered into a BLCC. Next, titanium nitride is deposited by CVD or PVD processing. A rapid thermal anneal step (RTA) then causes silicide formation. Tungsten is then deposited to fill the remaining opening in the BLCC. Depending upon the accuracy in the formation of the BLCC itself, it is possible for the BLCC to be shorted to other conducting layers. This is described below. In general, the BLCC can also be composed of tungsten, titanium/tungsten, aluminum, copper, a refractory metal silicide with aluminum, and a refractory metal silicide with copper.
As the size of the DRAM is reduced, the size of the active areas and the BLCCs available for contacts to reach the active areas are also reduced. Every process step has its own alignment limitations. While alignment is not exact between process steps, strict tolerances are required in order to accomplish a corridor that avoids a short between a contact that will be deposited in the BLCC and any other conductive materials (i.e., cell plate to active area). Hence, it is desirable to effectively isolate the contacts from the transistor and capacitor components while optimizing the space available to make the contacts.
The conventional methods of forming contacts between bit lines and active areas experience alignment problems in avoiding a short circuit between the electrically conductive bit line contact and the cell plate or storage node of a capacitor.